Hypothyroidism

THYROID GLAND

• The thyroid gland weighs approximately 20 g.
• Composed of two lobes joined by an isthmus
• The gland is closely affixed to the anterior and lateral aspects of the trachea, with the upper border of the isthmus located just below the cricoid cartilage.
• A pair of parathyroid glands is located on the posterior aspect of each lobe.
• The gland is innervated by the adrenergic and cholinergic nervous systems.

• The recurrent laryngeal nerve and external motor branch of the superior laryngeal nerve are in intimate proximity to the gland.
• Histologically the thyroid is composed of numerous follicles filled with proteinaceous colloid.
• The major constituent of colloid is thyroglobulin, an iodinated glycoprotein that serves as the substrate for thyroid hormone synthesis.
• The thyroid gland also contains parafollicular C cells that produce calcitonin

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Normal values of various thyroid parameters

Introduction

• Relatively common disease affecting 0.5%–0.8% of the adult population.
• Results in decreased production of thyroid hormones despite adequate or increased levels of TSH and accounts for 95% of all cases of hypothyroidism.
• In the United States is ablation of the gland by radioactive iodine or surgery.
• The second most common type of hypothyroidism is idiopathic and probably autoimmune in origin, with autoantibodies blocking TSH receptors in the thyroid.
• Hashimoto thyroiditis is an autoimmune disorder characterized by goitrous enlargement and hypothyroidism that usually affects middle-aged women.

Causes of thyroid disorders.

Primary

• Iodine deficiency.
• Thyroid ablation.
• Hashimoto thyroiditis
• Subacute thyroiditis
• Genetic abnormalities
• Goitrogenic food

(cabbage)

• Drugs like:

Lithium

Amiodarone

Anti thyroid agents

Beta- blockers

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Secondary

• Adenoma.
• Ablation therapy.
• Pituitary destruction.
• Hypothalamic dysfunction.

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Primary hypothyroidism

• Decreased production of thyroid hormones inspite of adequate or increased levels of TSH.
• TRH (thyrotropin releasing hormone) administration causes exaggerated TSH elevation.
• This most common type.

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Secondary hypothyroidism

• Reduced levels of freeT4, T3, and reduced TSH level.
• TRH( thyrotropin releasing hormone) stimulation confirms pituitary as cause, which shows absent or blunted reflex.

Euthyroid sick syndrome

• Abnormal thyroid function tests in critically ill patients with significant non thyroid illness.
• Low levels of T3,T4 and normal TSH level.
• With deterioration of the disease T3 and T4 level decreases further.
• Stress induced as a physiology response during surgery
• No treatment is needed.
• Serum TSH

>10 milli units/L implies hypothyroidism.

<5 milli units /L implies euthyroidism.

• General symptoms
• Skin
• Eyes
• Hematological findings
• Cardio vascular system
• Respiratory system
• Gastro intestinal system
• Reproductive system
• Neurological dysfunctions
• Hashimoto encephalopathy
• Myxedema coma
• Carpel tunnel syndrome
• Musculo skeletal system
• Metabolic abnormalities
• Drug clearance

Difference between primary and secondary hypothyroidism.

Classifying hypothyroidism by laboratory values

Treatment

• l-Thyroxine (levothyroxine sodium) is usually administered for the treatment of hypothyroidism.
• The first evidence of a therapeutic response to thyroid hormone is sodium and water diuresis and a reduction in the TSH level.
• In patients with hypothyroid cardiomyopathy, a measurable improvement in myocardial function is often achieved with therapy.
• Angina is uncommon in hypothyroidism, it can appear or worsen during treatment of the hypothyroid state with thyroid hormone. Medical management of such patients is particularly difficult

Management of Anesthesia

• Hypothyroid patients may be at increased risk for a number of reasons when undergoing either general or regional anesthesia.
• Airway compromise secondary to a swollen oral cavity, edematous vocal cords, or goitrous enlargement may be present.
• Decreased gastric emptying increases the risk of regurgitation and aspiration.
• A hypodynamic cardiovascular system characterized by decreased cardiac output, stroke volume, heart rate, baroreceptor reflexes.
• Intravascular volume may be compromised by surgical stress and cardiac-depressant anesthetic agents.

• Decreased ventilatory responsiveness to hypoxia and hypercarbia is enhanced by anesthetic agents.
• Hypothermia occurs quickly and is difficult to treat.
• Hematologic abnormalities such as anemia (25%–50% of patients) and dysfunction of platelets and coagulation factors (especially factor VIII)
• Electrolyte imbalances (hyponatremia), and hypoglycemia are common and require close monitoring intraoperatively.

PRE-OPERATIVE PERIOD

• Euthyroid state is ideal for surgical procedures
• For chronic thyroid disorder a preoperative thyroid function test is needed
• Reliable report - if it is less than 6 months
• Thyroid stimulating hormone (TSH) is the best to evaluate hypothyroidism.
• Surgical stress may precipitate myxedema or thyroid storm in untreated or severe cases
• Elective surgeries must be postpone until the patient is euthyroid.
• Emergency surgeries must be done after consultation with endocrinologist.
• Chest x-ray or CT is used to rule out tracheal or mediastinal involvement
• Continuation of drug on the day of surgery is important

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• In patients with no history of prior thyroid dysfunction but with present history if symptoms presents-TSH alone could be given.
• Full replacement dose of levothyroxine-
• 1.6micrograms/kg/day
• Elderly or those with coronary artery disease the initial dose-25 mcg daily increase every 2 to 6 weeks until euthyroid state
• Half life of the drug is 7 days

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INTRA-OPERATIVE PERIOD

• Increased risk when hypothyroid patient goes through general or regional anaesthesia
• Difficult intubation-Swollen oral cavity

-Edematous vocal cords

-Goitrous enlargement

• Aspiration risk and regurgitation risk due to decreased gastric emptying.

Cardiovascular changes

• hypodynamic circulation
• Decreased cardiac output.
• Decreased stroke volume.
• Decreased heart rate
• Decreased baroreceptor reflexes

INTRA-OPERATIVE PERIOD

Respiratory changes

• Enhanced suppression of ventilatory response to hypoxia and hypercarbia

Hematologic abnormalities

• Anemia 25%-50% of patients
• Platelet dysfunction and coagulation factor abnormalities (factor viii)
• electrolyte imbalances-hyponatremia

Metabolic demands

• Hypoglycemia is common
• Hypothermia has quicker onset which is Difficult to treat
• Increased neuromuscular excitability

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GENERAL ANESTHESIA

• Oral endotracheal tube intubation is commonly done.
• Rapid sequence induction or awake intubation done in case of difficult airway
• Inhalational agents may aggravate myocardial depression
• Pancuronium is the ideal neuromuscular blocker from cardiovascular

standpoint but careful dosing is needed due to reduced skeletal muscle activity

and reduced hepatic metabolism

• Controlled ventilation needed as spontaneous breathing may lead to hypoventilation
• Intraoperative hypotension is managed by pharmacological agents like

ephedrine, dopamine , epinephrine if unresponsive may need supplemental

steroid administration

• Dextrose in normal saline is preferred to avoid hypoglycemia and hyponatremia

Post operative period

• Myxedema coma is common in emergency cases
• Intravenous thyroid replacement therapy should be started
• intravenous L-thyroxine takes 10 to 12 days to yield peak basal metabolic rate
• Intravenous triiodothyronine is effective in 6 hours with peak metabolic rate seen in 36 to 72 hours
• Levothyroxine 300 to 500 mcg I.V or Levo-iodothyronine 25 to 50 mcg I.V is the initial dose.

• Hypothyroidism may be associated with decreased adrenal cortical function, steroid coverage with hydrocortisone or dexamethasone could be given.
• Milrinone phosphodiesterase inhibitor may be effective in the treatment of intraoperative myocardial depression
• Post operatively ,if still there is no ability to administer the drug enterally after 5 days, intravenous levo-thyroxine should be administered as 60% to 80% of the oral dose
• The hypothyroid group has a higher rate of gastrointestinal and neuropsychiatric complications post surgically.

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MYXEDEMA COMA

• Is a rare severe form of decompensated hypothyroidism
• Mostly seen in elderly women with chronic hypothyroidism
• Infection, trauma, cold and central nervous system depressant predispose hypothyroidism to myxoedema coma.
• Patient is not comatose but often needs mechanical ventilation
• Hypothermia of less than 27 degree centigrade is a cardinal feature with impaired thermoregulation by hypothyroidism

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Treatment of choice

intravenous L-thyroxine or L-triiodothyronine

• Intravenous fluid-glucose containing saline solution
• Thermoregulation
• Electrolyte imbalance correction
• Stabilization of cardiac and pulmonary function
• Vitals-heart rate, blood pressure, temprature improve 24 hours
• Relative euthyroid is achieved in 3 to 5 days
• Hydrocortisone 100-300mcg/day is given for adrenal insufficiency

• Management in the intensive care unit where proper ventilatory, electrolyte, and hemodynamic support can be given.
• Passive rewarming, broad spectrum antibiotic coverage and corticosteroids may be needed.
• The definitive treatment is thyroid hormone replacement administered as IV T4 200 to 500 mcg as an initial bolus followed by 50-100 mcg daily
• Few suggest addition of IV T3 ,10-25mcg every 8 hours if available.
• Rapid thyroid hormone replacement may precipitate myocardial infarction, hence caution should be exercised in those with underlying ischemic heart disease.
• Treatment of the precipitating cause like an infection is critical for rapid recovery.

PREGNANCY AND HYPOTHYROIDISM

• Pregnancy is a state of excessive thyroid stimulation.
• increase in thyroid size by 10% in iodide sufficient areas and 20-40% in iodide deficient regions
• Due to physiological and hormonal changes caused by pregnancy and human chorionic gonadotropin (HCG) the production of thyroxin (T4) and triiodothyronine (T3) increase up to 50%
• 50% increase in daily iodide need, while Thyroid-stimulating hormone (TSH) levels are decreased in first trimester.
• In an iodide sufficient area ,thyroid adaptations during pregnancy are tolerated, as stored inner iodide is sufficient.

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Clinical features:

OVERT HYPOTHYROIDISM

• Abortion
• Anemia pregnancy-induced hypertension
• Preeclampsia
• premature birth, low birth weight
• intrauterine fetal death
• increased neonatal respiratory distress
• infant neuro-developmental dysfunction
• placental abruption
• postpartum hemorrhage

SUBCLINICAL HYPOTHYROIDISM

• Higher chance of placental abruption
• preterm birth
• miscarriage
• gestational hypertension
• fetal distress
• severe pre-eclampsia
• neonatal distress
• diabetes in pregnant women

MEDICAL MANAGEMENT

• Pre-existing hypothyroidism, there is 30-50% increase in requirement of levothyroxine during the first trimester.
• Treated by iron supplements and TH four hours apart.
• levothyroxine should be started at a dose of 1-2 mcg/kg/day
• TSH levels should be reassessed 4-6 weeks following the dose change
• Treatment goal of TSH in the range of 0.5-2.5 mlU/L.
• Overt hypothyroidism diagnosed in pregnancy, T4 should be normalised as rapidly as possible by using two to three times the estimated final daily dose.

ANESTHETIC MANAGEMENT

• During pre-operative preparation, anxiolytic and sedatives should be avoided.
• Administration of antihistamines like ranitidine and oral sodium citrate solution along with metoclopramide are considered safe.
• Severe hypothyroidism should be managed with IV T3/T4
• Hypothermia should be prevented in the operation room as well as in the post operative period.
• Hypothyroidism is associated with platelet dysfunction. Arrangement of fresh frozen plasma or platelets is needed.
• epidural hematoma is a risk and presence of normal coagulation should be confirmed before regional anesthesia.

• Vasopressor response is normal for epinephrine but, decreased for phenylephrine.
• During surgical stress, hydrocortisone should be given.
• Regional anesthesia should be favoured over general anesthesia
• Nerve stimulators may not be useful clinically due to abnormal response to the peripheral nerve stimulator, due to expression of neuromuscular junction activities.

REFERENCES

• Miller's ANESTHESIA… 9th edition
• Stoelting's anesthesia and co-existing diseases.
• Iran j Reprod Med-review article-2015-Thyroid dysfunction and pregnancy outcomes.

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